It’s easy to pinpoint when the new horror movie Hereditary begins to get spooky. That occurs right off the bat when Steven Graham (Gabriel Byrne) tries to get his children ready to attend their grandmother’s funeral and encounters a strange resistance from son Peter (Alex Wolff) and daughter Charlie (Milly Schapiro) that goes beyond the usual sadness over a loss and grumbling over attending a formal church service. It’s also easy to pinpoint when Hereditary begins to go off the rails. That occurs when Steven’s wife Annie (Toni Collette) takes part in a grief support group later in the film and has a “chance” encounter with an overly friendly woman named Joan (Ann Dowd, who is always most dangerous in films in which she is overly friendly or concerned). From that moment on, a promising horror movie starts spiraling completely out of control.

Before that downhill slide occurs, writer/director Ari Aster treats the audience to a genuinely creepy first hour of Hereditary, concentrated on the Graham family who, aside from the harried in-over-his-head nice guy Steven, could be poster children for dysfunctionality. Annie earns her living designing intricately detailed miniature houses, an occupation that has turned into an obsession of redesigning and reworking. She also has lots of love/hate issues with her mother and her children, one night pouring paint thinner on the kids while they slept. She even tells Peter she never wanted him and tried to miscarry when pregnant with him. Amidst such loving maternal care, it’s not surprising that the children turned out flawed as well. In many ways Peter seems a normal teenager, albeit one obsessed with pot smoking, until one day he starts smashing his face into his desk at school. As for Charlie, she looks like Wednesday Addams’ creepier sister, and she likes to make bizarre clucking noises and cut the heads off dead birds with a pair of scissors.

It doesn’t take supernatural powers to guess that things aren’t going to go well for the Graham family and the beginning of the end occurs when Annie forces Peter to take Charlie to a high school party with him. Leaving a 12-year-old in the care of a not-very-mature older brother at a party of this nature is a recipe of disaster under any circumstances, but what transpires in Hereditary is the most shocking scene in the film and one of the most shocking I can recall in any movie in a long time. From that moment on, the disintegration of the Graham family simply gathers momentum, as evidenced by the clip below.

If director Aster had trusted his material enough to realize what the strengths of the film are, Hereditary could have been a powerful tragic melodrama about a family coming apart under stress. Indeed, the screenplay hints at that by having Peter’s English teacher talk to the class about the essence of tragedy. The key to the success of this storyline is Toni Collette in one of her best performances. Annie is trying to hold things together in the earlier part of the movie but fails, as we later learn, thanks to some supernatural obstacles in her course. But as viewers will soon be aware, she was destined to fall apart soon in any event. Watching the Graham family saga play out over another hour of screen time might have brought about comparisons with movies like Ordinary People.

However, Hereditary was not intended as a psychological horror story revolving around a family tragedy, but a supernatural story, one that the script drops hints about early on but then ladles the exposition on thick following the arrival of Ann Dowd’s Joan. It doesn’t require a genius to realize that the overly friendly Joan is up to no good, a suspicion that is confirmed shortly after she hosts a séance for the initially skeptical Annie that turns into a ghostly spectacle.

There is a rationale behind the various spooky events in Hereditary and it involves a demon named King Paimon who appears to be an actual demon according to prevailing demon lore. My guess is that, unlike the devil, Hereditary marks the first onscreen manifestation of King Paimon, whose supernatural abilities seem to consist of getting intelligent people to act incredibly stupid and causing good movies to jump the shark. The latter is certainly the case here, as the last half hour of this film is a mix of truly bizarre visuals and a couple of graphic moments that almost match the movie’s big shock early in the film. The only problem is that by throwing them all together in a confusing manner and tying that into a demonic force that sounds like a secondary character from Game of Thrones, Hereditary ceases to be really creepy and instead becomes head-scratchingly weird.

I think that the problem the film faces isn’t the direction by Aster so much as his script. He does create some genuinely creepy moments, avoiding the typical jump scare fallbacks so many other directors employ, in favor of a mounting mood of menace that makes the forst hour of the film surprisingly effective. But it’s telling that Aster’s previous experience has been limited to shorts of 30 minutes or less. As far as directing a horror scene in a short as opposed to a feature length movie, there is no real difference. But a two-hour horror movie requires a two-hour script, and Aster falls an hour short here. His inspiration for Hereditary appears to have been Rosemary’s Baby, but, while Roman Polanski’s screenplay mirrored the Ira Levin source novel quite closely, Aster has no such inspiration. As a result, instead of a tightly constructed, very logical (if you accept the main premise) film like the Polanski classic, Aster’s film winds up feeling overstuffed and bloated with goings on that go past scary to screwy.

Still, Hereditary marks an impressive feature film debut for Ari Aster, who has demonstrated a knack for figuring out what is unsettling and how to scare people through a gradually mounting series of offputting events rather than simply go for the fright immediately. When he works with a better script, he could become one of the best horror directors around. Unfortunately, Hereditary does not have that script, and it ends up more ludicrous than terrifying.